Sunil Chintalapati

Experimental and Numerical Investigation of Liquid Slosh Behavior Using Ground-Based Platforms

Liquid-propellant slosh events occurring during orbital maneuvers of a rocket’s upper stage may adversely affect vehicle performance. Mission planners require accurate and validated simulation tools to understand and predict the effects of slosh on the intended trajectory of the vehicle, as well as for propellant and tank thermal management. A coupledrigid-bodyandfluid-dynamicsnumerical tool is presentedwhich canbeused topredict the effect of internalfluid slosh on a tank’s trajectory. To benchmark the numerical tool, a novel experimental framework is presented which examines the influence of liquid slosh on the trajectory of moving tanks with multiple degrees of freedom. The motion history of the tank is measured along with synchronized camera images of the liquid distribution within the tank. The acquiredexperimentaldataareused toassess the accuracyof thenumerical tool.Thepredictions fromthenumerical tool are in excellent agreement with the experimentally measured data over a wide range of tank motion profiles.

Update on SPHERES Slosh for Acquisition of Liquid Slosh Data aboard the ISS

A current problem that severely affects the performance of spacecraft is related to slosh dynamics in liquid propellant tanks under microgravity conditions. The confidence in computational fluid dynamics (CFD) predictions are low because of the lack of benchmark against experimental data. The goal of the SPHERES Slosh Experiment (SSE) is to acquire long-duration, low-gravity liquid slosh data aboard the International Space Station. The proposed experimental platform consists of a tank, partially filled with fluid, cameras, and inertial sensors to monitor the fluid distribution in the tank. Currently the SSE has passed NASA’s Critical Design Review (CDR) and Phase 0/I-II Flight Safety Review (FSR). The manufacture and qualification testing of the flight article has been completed. The Slosh payload is scheduled to be launched in December 2013 with on-orbit experiments to begin shortly thereafter.

Parametric Study of a Propellant Tank Slosh Baffle

A current problem that severely affects the performance of spacecraft is related to slosh dynamics in liquid propellant tanks under microgravity conditions. Accurate prediction of the slosh dynamics is critical for successful mission planning and may impact vehicle control and positioning during rendezvous, docking, and reorientation maneuvers. The purpose of this work is to assess the performance of various sloshmitigating baffle designs and configurations using computational fluid dynamics. This work develops metrics, including wall wetting, peak slosh amplitude, and bulk fluid motion, to assess the relevance of a particular baffle geometry and placement within the tank for a prescribed bulk fluid motion over a range of acceleration levels. The twoand three-dimensional studies are used to assess the slosh model’s sensitivity to grid resolution, laminar versus turbulent flow models, and Bond number scaling. The results are used to develop a foundation on which to build a full six-degree-of-freedom dynamic mesh model, allowing for fluidforce interaction with a propellant tank, which will be benchmarked against low-gravity slosh flight data.

Effect of Isogrid-Type Obstructions on Thermal Stratification in Upper-Stage Rocket Propellant Tanks

Analytical models for propellant thermal stratification are typically based on smooth wall flow correlations. However, many propellant tank walls have a mass-saving isogrid, which alters the boundary layer. This work investigates the boundary-layer behavior over walls with obstruction elements that are representative of isogrid or internal stiffener rings. The experimental studies reveal that the thickness of the velocity boundary layer over an isogrid wall is more than 200% thicker than a smooth wall at full-scale upper-stage tank Reynolds numbers. For buoyancy-induced free convection flows, the computational-fluid-dynamics models demonstrate that the velocity boundary layer over a wall lined with obstruction elements may be thicker or thinner than the equivalent boundary layer over a smooth wall, whereas the thermal boundary layer is always thicker for the rough wall. A Rayleigh number scaling analysis is presented for a range of fluids, tank and obstruction sizes, heat loads, and acceleration levels. W...

Design of an Experimental Platform for Acquisition of Liquid Slosh Data aboard the International Space Station

Liquid propellant slosh occurring during orbital maneuvers of a rocket’s upper-stage may adversely affect vehicle performance. During orbital maneuvers the acceleration levels of an upper-stage vary substantially and significant liquid sloshing events may occur which can affect the intended trajectory of the vehicle as well as influence propellant and tank thermal management. Mission planners require accurate and validated simulation tools to understand and predict the effects of slosh on a mission and therefore there is a need to better understand and model liquid slosh in micro-gravity. This work aims to bridge the gap of missing data by designing a Slosh Platform capable of acquiring long-duration low-gravity slosh data on the International Space Station. The slosh data acquired will facilitate calibration and validation of CFD models. The proposed experimental platform consists of a tank partially filled with water, inertial measurement units to measure the dynamics of the system, as well as cameras to image the fluid distribution in the tank. The Slosh Platform utilizes the existing ISS SPHERES apparatus, which will be used to maneuver the tank through a variety of trajectories. To mimic the motions of actual upper-stage rocket maneuvers, a scaling analysis based on relevant nondimensional parameters is performed and applied to the design and operation of the Slosh platform onboard ISS. The proposed motion profiles to be conducted by the Slosh Platform on ISS were verified using a CFD tool to assess the magnitude of fluid slosh and its impact on the attitude change of the platform. The CFD tools predict that the liquid slosh within the tank of the Slosh Platform will cause the trajectory of the apparatus to deviate in a measurable way as compared with a rigid, non-sloshing system. Currently the experimental design has passed NASA’s Critical Design Review and manufacture of the first test article is in progress. The Slosh experiment is scheduled to fly to the ISS in July of 2013 with on-orbit experiments to begin shortly thereafter.

Effect of Isogrid Roughness on Thermal Stratification

Payloads requiring insertion into high altitude orbits are delivered using the upper stages of chemical rockets (ex., Delta and Atlas classes) normally employing cryogenic propellants. During the transfer period between orbits, the upper stage may coast for several hours during which time the thermodynamic state of the propellants may vary due to solar heating. At the conclusion of the coast phase, and in preparation for orbital insertion of the payload, the propellants must be within a narrowly defined range of temperature and pressure for the engine to resume operation. Buoyancy-driven thermal stratification of the propellant is one of the critical mechanisms taking place during this coast phase. Traditional stratification models are based on velocity and temperature correlations developed for flow along smooth vertical walls. In contrast, actual propellant tanks may have a mass-reducing Isogrid internal surface over which the velocity and temperature profiles differ significantly from smooth-wall correlations. A preliminary study to investigate the impact of Isogrid on the boundary layer has shown that the thickness of the layer adjacent to the wall in a forced freestream flow is substantially thicker (150-700%) than the equivalent flat plate boundary layer thickness. Furthermore, the flow is highly turbulent with many recirculation zones suggesting that the classical idea of a boundary layer may not exist over such geometries. Further investigation has shown that the effects of the Isogrid on thermal stratification can either suppress or enhance stratification relative to smooth tanks and is dependant on the roughness size and tank conditions.

Thermal Stratification Modeling Improvements for Isogrid-Lined Tanks

Payloads requiring insertion into high altitude orbits are delivered using the upper stages of chemical rockets (ex., Delta and Atlas classes) normally employing cryogenic propellants. During the transfer period between orbits, the upper stage may coast for several hours during which time the thermodynamic state of the propellants may vary due to solar heating. At the conclusion of the coast phase, and in preparation for orbital insertion of the payload, the propellants must be within a narrowly defined range of temperature and pressure for the engine to resume operation. Buoyancy-driven thermal stratification of the propellant is one of the critical mechanisms taking place during this coast phase. Traditional stratification models are based on velocity and temperature correlations developed for flow along smooth vertical walls. In contrast, actual propellant tanks may have a mass-reducing Isogrid internal surface over which the velocity and temperature profiles differ significantly from smooth-wall correlations. A preliminary study to investigate the impact of Isogrid on the boundary layer has shown that the thickness of the layer adjacent to the wall in a forced freestream flow is substantially thicker (150-700%) than the equivalent flat plate boundary layer thickness. Furthermore, the flow is highly turbulent with many recirculation zones suggesting that the classical idea of a boundary layer may not exist over such geometries.

Non-Dimensional Parameterization of Tank Purge Behavior

Many launch support processes use helium gas to purge rocket propellant tanks and fill lines to rid them of hazardous contaminants. As an example, the purge of the Space Shuttle’s External Tank used approximately 1,100 kg of helium. With the rising cost of helium, initiatives are underway to examine methods to reduce helium consumption. Current helium purge processes have not been optimized using physics-based models, but rather use historical ‘rules of thumb’. To develop a more accurate and useful model of the tank purge process, computational fluid dynamics simulations of several tank configurations were completed and used as the basis for the development of an algebraic model of the purge process. The computationally efficient algebraic model of the purge process compares well with a detailed transient, three-dimensional CFD simulation as well as with experimental data from two External Tank purges.

Enhanced Numerical Modeling in Simulation of a Generic Propellant Tank Slosh Baffle

The slosh dynamics in cryogenic fuel tanks under microgravity is a pressing problem that severely affects the reliability of spacecraft launching. An accurate prediction of the slosh is critical for successful mission planning and may influence vehicle control and positioning during rendezvous, docking, and reorientation maneuvers. This paper defines a novel method to assess this problem by coupling capabilities of ANSYS FLUENT and MathWorks-MATLAB. While FLUENT solves the fluid dynamics aspect of problem, MATLAB solves for dynamics part of motion profile and update CFD with new inputs every new time step. Experiments performed on the Florida Tech 2-DOF motion table provided a basis for validating the new method implemented in CFD simulation. Present work focuses on enhancing the capabilities of CFD tool by reducing coding complexities due to inclusion of 6-DOF dynamic mesh. The idea is to incorporate resulting forces and other multiple inputs into FLUENT through MATLAB rather than a user defined function. MATLAB can independently compute the force feedback due to fluid forces and tank motion and incorporate it into simulation in the next incremental time-step.

Modeling of Ullage Collapse Within Rocket Propellant Tanks at Reduced Gravity

Orbital maneuvers of upper-stage rockets may generate propellant slosh waves and propellant breakup into droplets within the oxidizer and fuel tanks. When many droplets are created, increased propellant evaporation may occur and lead to a significant reduction in ullage gas temperature and pressure, known as ullage collapse. This work presents a new method that uses a numerical tool to predict the droplet distribution created during a maneuver, and subsequently uses an analytical model to calculate the droplet evaporation and consequent change in tank temperature and pressure. This new hybrid method uses less computational time than a detailed computational fluid dynamics model and is capable of providing mission planners with an improved tool to assess the impact of propellant evaporation on the requirements of additional helium mass for tank repressurization. The new method is applied to predict evaporation and the consequent ullage collapse in an upper-stage propellant tank undergoing simulated orbital...